Electrochemical gas sensor, filter and methods

ABSTRACT

The invention relates to an electrochemical gas sensing apparatus for sensing one or more analytes, such as NO 2  and/or O 3 , in a sample gas and a method of using same. The apparatus uses Mn 2 O 3  as a filter for ozone. The Mn 2 O 3  may take the form of a powder which may be unmixed, mixed with various PTFE particles sizes, formed into a solid layer deposited onto a membrane and/or pretreated with NO 2 .

FIELD OF THE INVENTION

The invention relates to an electrochemical gas sensing apparatus forsensing a gaseous analyte, characterised by a filter for ozone andmethods for making and using such apparatus.

BACKGROUND TO THE INVENTION

In order to measure atmospheric pollutants which may be present in airat low concentrations e.g. less than 5 or 10 ppb, it is necessary toprovide air monitoring instruments which are sensitive, specific andwhich perform reliably for a long period of time.

One way to provide specificity is to provide a filter between an inletinto electrochemical gas sensing apparatus and the sensing electrode,where the filter is selected to remove one or more gas species which mayotherwise interfere with measurements of the target analyte gas.

It has been proposed to use MnO₂ as a filter material in aconductometric sensor to filter O₃ out of ambient air (Viricelle J P etal., Materials Science and Engineering C, Elsevier Science S. A., CH,vol. 26, no. 2-3, pages 186-195). EP 2975390 (Alphasense Limited)proposed the use of an MnO₂ filter adjacent one of two carbon sensingelectrodes which receive gas in parallel, in a sensor for the detectionof NO₂ and/or O₃, without interference from SO₂, CO, NO, NH₃ and H₂.

Although MnO₂ is useful as a filter material to filter O₃ out of ambientair, for example in a sensor for the detection of NO₂ and/or O₃, it cancreate a problem with cross-sensitivity to NO. Without wanting to bebound by theory, we hypothesize that NO reacts with MnO₂ to form NO₂which then reacts at the sensing electrode.

Accordingly, the invention seeks to provide an improved sensor for thedetection of at least one gaseous analyte. Some embodiments relate tothe detection of NO₂ and/or O₃.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is providedelectrochemical gas sensing apparatus for sensing at least one gaseousanalyte and comprising a housing having an inlet, a sensing electrodeand an ozone filter interposed between the sensing electrode and theinlet, wherein the ozone filter comprises Mn₂O₃.

The at least one gaseous analyte is sensed (where present) in a gassample, which may be air (typically adjacent the sensing apparatus). Theozone filter is interposed between the sensing electrode and the inletsuch that the ozone filter filters gas (from a gas sample) receivedthrough the inlet before it reaches the sensing electrode. Typically,there is a gas path (along which gas flows and/or diffuses in use)extending between the inlet and the sensing electrode, and the ozonefilter is located in the gas path. It may be that all gas receivedthrough the inlet passes through the ozone filter before it reaches thesensing electrode.

The at least one gaseous analyte may be NO₂ and/or O₃. Theelectrochemical gas sensing apparatus may be electrochemical gas sensingapparatus for sensing NO₂ and/or O₃. The electrochemical gas sensingapparatus may have one or more outputs which output an electrical signal(e.g. current, or potential or a digital value) related to theconcentration of the one or more analytes. The sensing electrode (andthe second sensing electrode where present) is therefore typically anelectrode at which NO₂ is reducible. Our further discussion will focuson the detection of NO₂ and/or ozone. However, the at least one analytemay comprise another gas, for example the least one analyte may be orcomprise SO₂.

Mn has a degree of oxidation of IV in MnO₂ and III in Mn₂O₃. Mn₂O₃ istherefore a less strong oxidising agent and has distinct chemistry toMnO₂. Despite this we have surprisingly found that it is useful as anozone filter in a practical sensor for detecting NO₂ and/or O₃ becauseof the possibility of forming a sensor with a low cross-sensitivity toNO. It may be that the gas sample comprises NO. It may be that the gassample comprises a higher concentration of NO than NO₂. This commonlyoccurs in atmospheric gas monitoring and means that NO cross-sensitivityis particularly important.

It may be that the ozone filter comprises (or consists of) Mn₂O₃ powder.

It may be that the Mn₂O₃ (e.g. Mn₂O₃ powder) is NO₂ treated. Thus, theMn₂O₃ (e.g. Mn₂O₃ powder) may have been treated with NO₂. The Mn₂O₃(e.g. Mn₂O₃ powder) may as a result have NO₂ adsorbed to the surfacethereof. The Mn₂O₃ (e.g. Mn₂O₃ powder) may be substantially NO₂saturated. We have found that this improves the reliability of readingsof measurements of the concentration of NO₂ in sensing apparatus fordetecting NO₂ and/or O₃. It may be that the (treated) Mn₂O₃ (e.g. Mn₂O₃powder) has at least 10¹² molecules of NO₂ adsorbed thereto per cm² ofsurface.

It may be that at least 2×10¹² molecules of NO₂ are adsorbed per cm² ofsurface. The surface area is typically as determined by BET(Brunauer-Emmett-Teller (BET) theory).

It may be that at least 2, or at least 3, or at least 4 moles of NO₂ areadsorbed per gram of Mn₂O₃.

The ozone filter may comprise Mn₂O₃ powder mixed with binder.

Surprisingly, we have found that the decrease in sensitivity at lowtemperature is reduced where the Mn₂O₃ (e.g. Mn₂O₃ powder) is mixed withbinder than if the filter comprises only Mn₂O₃ (e.g. Mn₂O₃ powder).Furthermore, NO cross sensitivity may be reduced when Mn₂O₃ (e.g. Mn₂O₃powder) is mixed with binder rather than unmixed. Without wanting to bebound by theory, we hypothesize this can reduce the rate at which NO maybe converted to NO₂ by the Mn₂O₃ (e.g. Mn₂O₃ powder) within the filterwhile still providing sufficient ozone filtering capacity.

Typically, the Mn₂O₃ powder makes up less than 50%, less than 25%, lessthan 20% or less than 15% by mass of the combined mass of the Mn₂O₃ andthe binder (in the mixture of Mn₂O₃ and binder).

Typically, the Mn₂O₃ powder makes up greater than 5% or greater than 8%of the combined mass of the Mn₂O₃ and the binder (in the mixture ofMn₂O₃ and binder).

For example, the Mn₂O₃ powder may make up 8% to 20%, 8-17% or 8-15% ofthe combined mass of the Mn₂O₃ and the binder (in the mixture of Mn₂O₃and binder).

It may be that the Mn₂O₃ powder is mixed with binder such as to reducethe available Mn₂O₃ surface area per unit of surface area of the inletat least 4-fold or at least 10-fold.

It may be that the Mn₂O₃ powder is mixed with binder such that theavailable Mn₂O₃ surface area, as measured by BET analysis, per unit ofcross-sectional area of the filter (perpendicular to the gas paththrough the filter) is less than 0.8 m² per cm², or less than 0.5 m² percm², or less than 0.3 m² per cm². It may be that the Mn₂O₃ powder ismixed with binder such that the available Mn₂O₃ surface area, asmeasured by BET analysis, per unit of cross-sectional area of the filteris at least 0.015 m² per cm² at least 0.02 m² per cm², at least 0.025 m²per cm², at least 0.05 m² per cm², or at least 0.1 m² per cm².

The binder may comprise particles. The particles may bepolytetrafluoroethylene (PTFE) particles. The binder may for example, bealumina or a silicate. The binder may assist by occupying volume so thatthe active surface area of the Mn₂O₃ powder per volume of gas space inthe filter is reduced.

The Mn₂O₃ powder may be in the form of particles with a mean diameter of5 to 100 microns. The Mn₂O₃ powder may be in the form of particles witha mean diameter of 25 to 75 microns.

The binder particles may have a mean diameter of 25 to 1500 microns. Thebinder particles may comprise PTFE particles having a mean diameter ofgreater than 100 but less than 1500 microns, or greater than 500 butless than 1500 microns, for example.

The Mn₂O₃ powder may coat the binder particles. The binder particles maybe coated with Mn₂O₃ powder.

Where the ozone filter comprises a mixture Mn₂O₃ powder and aparticulate binder, the mixture of particles is preferably provided in achamber which is filled with the mixture. This reduces separation of theMn₂O₃ powder and the binder particles.

Preferably, the Mn₂O₃ powder has a purity of at least 98%, or at least99%.

It may be that the ozone filter comprises a solid layer, the solid layercomprising Mn₂O₃. The ozone filter may comprise both said Mn₂O₃ powder(e.g. located in a chamber) and a solid layer, the solid layercomprising Mn₂O₃. The solid layer is typically a solid microporous layercomprising Mn₂O₃.

The solid microporous layer may be formed on a gas porous support, forexample on a side facing the sensing electrode or a side facing awayfrom the sensing electrode. It may be that the solid microporous layeris formed on a gas porous support which also supports the sensingelectrode, e.g. on opposite sides.

The solid microporous layer may be formed only of Mn₂O₃. The solidmicroporous layer may comprise at least 50% or at least 75% or at least85% of by mass Mn₂O₃. The solid microporous layer may comprise binder,for example PTFE (e.g. PTFE particles). It may be that at least 90% orat least 95% of the solid microporous layer is Mn₂O₃ mixed with binder.The solid microporous layer may for example comprise at least 5% ofbinder by mass or at least 10% of binder by mass. The solid microporouslayer may for example comprise less than 25% of binder by mass or lessthan 15% of binder by mass.

Typically, the Mn₂O₃ in the solid microporous layer has a purity of atleast 98%, or at least 99%. The Mn₂O₃ in the solid layer may be NO₂treated as above. The solid layer may be formed from Mn₂O₃ powder whichhas been NO₂ treated as above.

It may be that the sensing electrode (and optionally the second sensingelectrode, where present) is a carbon electrode. Carbon electrodes areadvantageous in that NO₂ is reducible to give a current but interferencefrom SO₂, CO, NO, NH₃ and H₂ is generally inhibited. The sensingelectrode may however be another type of electrode, for example the(first) sensing electrode may comprise gold (for example it may be agold or binary gold/gold oxide electrode). In the case of sensing SO₂,for example, the sensing electrode may be a gold and rutheniumelectrode.

In some embodiments the said sensing electrode is a first sensingelectrode and there is further provided a second sensing electrode.Typically, ozone is filtered from the sample gas received by the firstsensing electrode by the ozone filter but ozone is not filtered from thesample gas received by the second sensing electrode. Thus, it may bethat the gas sensing apparatus comprises a second sensing electrode,wherein the second sensing electrode is in gaseous communication with agas sample (typically surrounding air) without an intervening ozonefilter.

The second sensing electrode is used to obtain a measurement including asignal due to any ozone which is present in a gas sample. This signalcan be compared with the signal from the first sensing electrode (whichis in contact with gas which has been filtered by the ozone filter). Thefirst sensing electrode and the second sensing electrode may be providedin separate electrochemical sensors. The electrochemical sensors mayboth be provided in the said housing. However, the second sensingelectrode may be provided in a separate sensor body.

According to a second aspect of the invention there is provided a methodof forming an electrochemical gas sensing apparatus for sensing at leastone gaseous analyte, the method comprising providing a housing having aninlet and a sensing electrode and providing an ozone filter interposedbetween the sensing electrode and the inlet, wherein the ozone filtercomprises Mn₂O₃.

Thus, the sensing electrode is in gaseous communication with a gassample, typically surrounding gas (e.g. the surrounding atmosphere)through the ozone filter.

The at least one gaseous analyte may comprise or be NO₂ and/or ozone.The gaseous analyte may be SO₂.

The ozone filter may comprise Mn₂O₃ powder. The method of forming anelectrochemical gas sensing apparatus may comprise the step of treatingthe Mn₂O₃ (e.g. Mn₂O₃ powder) with NO₂.

As a result, NO₂ adsorbs to the surface of the Mn₂O₃ (e.g. Mn₂O₃powder). The Mn₂O₃ (e.g. Mn₂O₃ powder) is preferably treated with NO₂until the Mn₂O₃ (e.g. Mn₂O₃ powder) is saturated with NO₂, i.e. untilfurther treatment with NO₂ does not have a significant effect. It may bethat the Mn₂O₃ (e.g. Mn₂O₃ powder) is treated with NO₂ at aconcentration of at least 10 ppm for at least 30 mins. It may be that atleast 10¹², or at least 2×10¹² molecules of NO₂ are adsorbed per cm² ofsurface of Mn₂O₃. It may be that sufficient NO₂ is provided that atleast 1 or at least 1.5 moles of NO₂ are adsorbed per gram of Mn₂O₃.

It may be that the Mn₂O₃ powder is mixed with binder particles. Thefilter may thereby comprise a mixture of Mn₂O₃ powder and binderparticles. The Mn₂O₃ powder may coat the binder particles. Inembodiments where the Mn₂O₃ is treated with NO₂, the treatment with NO₂typically takes place after the Mn₂O₃ powder is mixed with binder, butthis is not essential.

It may be that the ozone filter comprises a solid microporous layer, thesolid microporous layer comprising Mn₂O₃. The ozone filter may compriseboth said Mn₂O₃ powder (e.g. located in a chamber) and a solidmicroporous layer, the solid microporous layer comprising Mn₂O₃. Themethod may comprise gas passing through both a region of the ozonefilter comprising Mn₂O₃ powder and a region of the ozone filtercomprising a solid microporous layer, the solid microporous layercomprising Mn₂O₃ powder (optionally in that order) to reach the sensingelectrode.

The solid microporous layer may be formed only of Mn₂O₃. The solidmicroporous layer may comprise binder, for example PTFE (e.g. PTFEparticles). The solid microporous layer may for example comprise atleast 5% of binder by mass or at least 10% of binder by mass. The solidmicroporous layer may for example comprise less than 25% of binder bymass or less than 15% of binder by mass.

The solid layer may comprise Mn₂O₃ which has been treated with NO₂ asabove. The solid layer may be formed from Mn₂O₃ powder which has beenNO₂ treated as above, optionally after mixing with one or more othercomponents, for example binder (such as PTFE).

The solid microporous layer may be formed by compressing Mn₂O₃ powder(optionally after treatment with NO₂ as above and/or mixing withbinder).

The method may comprise providing a second sensing electrode in gaseouscommunication with a gas sample (typically surrounding air) without anintervening ozone filter. The signals from the (first) and secondsensing electrodes may be compared to measure the concentration of ozoneor to cancel out the effect of ozone on the measurement of another gas(e.g. NO₂ or SO₂).

The first (and second, if present) sensing electrodes may for example becarbon electrodes or electrodes comprising gold (e.g. gold/gold oxideelectrodes).

The invention extends to a method of sensing at least one gaseousanalyte (for example, NO₂ and/or ozone, or SO₂ and/or ozone) comprisingforming electrochemical gas sensing apparatus according to any one ofclaims 1 to 7, or by the method of any one of claims 8 to 12 andbringing the inlet into gaseous communication with a gas sample.

The gas sample may be ambient air around the gas sensing apparatus.

Thus, the invention extends to a method of sensing a gaseous analyte(for example NO₂ and/or ozone, or NO₂ and/or ozone) comprising formingan electrochemical gas sensing apparatus by the method set out above andbringing the apparatus into gaseous communication with a gas sample(e.g. the surrounding air) such that both the first and second sensingelectrodes reduce any of a gaseous species (e.g. NO₂ or SO₂) in the gassample, where present, and the ozone filter removes ozone, wherepresent, from sample gas reaching the first sensing electrode but notthe second sensing electrode, such that the difference in the signals(typically currents) in the first and second sensing electrodes isrepresentative of the concentration of ozone in the gas sample, andthereby determining from the signals (typically currents) from the firstand second sensing electrodes the concentration of NO₂ and/or ozone inthe gas sample. This may also be applied to the detection of another gas(e.g. SO₂) and/or ozone.

The method may comprise sensing NO₂ and/or ozone in a gas sample whichcomprises a higher concentration of NO than NO₂. In this case, a lowcross sensitivity to NO is of particular importance.

By a carbon electrode we refer to an electrode containing carbon as anelectrode active material. Where the (first) sensing electrode (andsecond sensing electrode, where present) are carbon electrodes, thecarbon of the (or each) carbon electrode may be activated carbon,amorphous carbon, graphite, graphene (the graphene may be infunctionalised form, such as COOH-functionalised), glassy (or vitreous)carbon, fullerene, carbon nanotubes (including single walled, doublewalled and multi-walled carbon nanotubes) or boron-doped diamond (BDD)or some other suitable allotrope of carbon. It may be that the carbon ofthe (or each) carbon electrode is in the form of graphite, graphene,carbon nanotubes or glassy carbon. In one embodiment the carbon isgraphite. In one embodiment, the carbon is chosen from single walled ordouble walled carbon nanotubes. In one embodiment, carbon is the onlyelectrode active material in the electrode. In one embodiment, carbon isthe main electrode active material in the electrode, i.e. over 50 weight%, preferably over 80 weight % of the total electrode active materialpresent in the each of the first and second sensing electrodes iscarbon. It may be that the first sensing electrode and the secondsensing electrode (where present) comprise the same type of carbon. Inthis case, they may be the same in dimension and amount of electrodeactive material, so that each of the first sensing electrode and thesecond sensing electrodes provides a similar electrochemical response toNO₂. Alternatively the second sensing electrode (where present) can bemade from a different type of carbon or have a different composition ofelectrode active material to that of the first sensing electrode.

In the embodiments with first and second sensing electrodes, the secondsensing electrode may be an electrode at which both a first gas (e.g.NO₂) and O₃ are reducible. By this is meant that the first gas (e.g.NO₂) and O₃ can be reduced at the electrode. Thus, in operation, any ofthe first gas (e.g. NO₂) or O₃ reaching the second sensing electrode ofthe gas sensing apparatus of the invention will react to generate acurrent. In this case, the first sensing electrode is an electrode atwhich first gas (e.g. NO₂) is reducible. O₃ may also be reducible atthis electrode. However, any significant amount of ozone in the samplegas would be filtered out by the ozone filter. Where the sensingapparatus is for sensing NO₂ and/or O₃ and the first and second sensingelectrodes are carbon electrodes, we have found that carbon electrodesare very specific to NO₂ and O₃ and have low cross-sensitivities forother gases, i.e. NO₂ and O₃ will oxidise at the electrodes generatingcurrent reading whereas other gases such as NO and SO₂ will not. The useof such electrodes ensures a high level of accuracy of the measurementin the gas sensing apparatus of the invention. This is particularlyimportant for the measurement of NO₂. Thus the gas sensing apparatus ofthe invention can be used to provide a highly accurate measurement ofthe amount of NO₂ in the atmosphere, for example in urban areas wherethe amount of NO₂ in the atmosphere is regulated by law. The ability ofthe gas sensing apparatus of the invention to make a highly accuratemeasurement of ozone (the amount of which in the atmosphere is alsoregulated by law) at the same time makes the apparatus a very versatileenvironmental monitoring instrument. This, the apparatus may be for thedetection of the amount of NO₂ and O₃ (or for example SO₂ and O₃) in asample gas, such as air, and is particularly useful in applicationswhere an accurate determination is essential for health and safetyreasons. In particular, the method of the invention provides a veryaccurate indication of NO₂.

In embodiments with first and second sensing electrodes, the gas sensingapparatus is configured so that, in operation, the second sensingelectrode is exposed to the sample gas in parallel with the ozonefilter. In other words, the second sensing electrode and the firstsensing electrode are effectively exposed to the sample gas at the sametime, the first sensing electrode being exposed to the sample gas afterit has passed through the ozone filter. The ozone filter is configuredand positioned such that it does not cause a significant delay in thetransport of the sample gas to the first sensing electrode. The secondsensing electrode and the ozone filter can be described as beingsimultaneously in direct communication with the sample gas, i.e. thesecond sensing electrode and the ozone filter are not exposed to thesample gas in series. This can be achieved by having the first andsecond sensing electrodes positioned in close proximity or adjacent toeach other. By close proximity is meant that they are both present inthe same 0.5 cm³ or between 1 and 5 mm apart, for example. The gassensing apparatus of the invention can thus be small in size andcompact.

The gas sensing apparatus of the invention is an electrochemical gassensing apparatus and may, for example, be an amperometric gas sensingapparatus. The (first) sensing electrode (and the second sensingelectrode, if present) may be working electrodes (of an amperometricsensor). In such apparatus each working electrode is associated with acounter electrode, a reference electrode and an electrolyte, i.e. theworking electrode is connected conductively (electrochemically) to thecounter electrode and reference electrode through the electrolyte. Thecounter electrode, reference electrode and electrolyte can be the sameor different for each working electrode. A reduction or oxidationreaction at a working electrode generates a current between it and itscounter electrode. The principle of amperometry is based on themeasurement of this current. The reference electrode is used to set theoperating potential of the working electrode or to bias the workingelectrode for best performance. The gas sensing apparatus can comprise apotentiostat circuit for this purpose. The gas sensing apparatus ispreferably diffusion limited, with a diffusion barrier (such as acapillary or a porous membrane) controlling access of the sample gas tothe working electrodes. The combination of electrodes operating should,in principle, have sufficient activity to maintain capillary diffusionlimited behaviour. In other words the electrodes must be capable offully consuming the capillary-limited flux of the target gas reachingit.

In some embodiments, the first and second working electrodes do notshare common counter and reference electrodes, or a common electrolyte.Each of the first and second electrodes is associated with its owncounter electrode, reference electrode and electrolyte. In thisembodiment the gas sensing apparatus of the first aspect of theinvention further comprises a first counter electrode, a first referenceelectrode, a first body of electrolyte and a second counter electrode, asecond reference electrode and a second body of electrolyte. In thisembodiment, each of first working, counter and reference electrodes arein electrochemical contact with each other through the first body ofelectrolyte and each of second working, counter and reference electrodesare in electrochemical contact with each other through the second bodyof electrolyte. In this embodiment of the invention, the gas sensingapparatus can comprise two individual gas sensors, one of which hasfirst working, reference and counter electrodes and a first electrolyte,the other having second working, reference and counter electrodes, asecond electrolyte and an ozone filter adjacent the first workingelectrode. The sensors can be identical, except for the presence of theozone filter in one of them. The sensors can be situated and coupled toeach other on the same circuit board.

In one embodiment, the first and second working electrodes areassociated with and share a common counter electrode, a common referenceelectrode and a common electrolyte. In other words, the gas sensingapparatus of the first aspect of the invention further comprises onecounter electrode, one reference electrode, and one body of electrolyte,with the first, second, counter and reference electrodes being inelectrochemical contact with each other through said body ofelectrolyte. In this embodiment of the invention, a particularly compactgas sensing apparatus is achievable, due to the relatively small numberof components.

The reference and counter electrodes used in the gas sensing apparatusof the invention can be made of various electrode active materials whichinclude carbon, gold, gold alloys, Pt, and Pt alloys. The platinum canbe in the form of platinum oxide which includes platinum black (Ptb) andthe gold can be in the form of gold oxide which includes gold black. Thecarbon and a carbon electrode are as described above for the first andsecond working electrodes. The counter electrodes can be the same ordifferent from the reference electrodes. In one embodiment, the first,second or common reference electrode is chosen from carbon, gold, goldalloy, Pt, and Pt alloy electrodes and the first second or commoncounter electrode is the same as or different from the respective first,second or common reference electrode. In one embodiment, the first,second or common reference electrode is chosen from carbon, gold, goldalloy, Pt, and Pt alloy electrodes and the first second or commoncounter electrode is a Ptb electrode. Other combinations of referenceand counter electrodes include: carbon reference electrode and platinumblack counter electrode; and gold reference electrode and platinum blackor gold counter electrode.

In one embodiment, the working electrode, or each of the first andsecond working electrodes, where applicable, has an additional workingelectrode associated with it, the additional working electrode beingincorporated in the gas sensing apparatus such that it is not exposed tothe sample gas. The additional electrode is buried within the sensorbody and is used to generate a signal for correcting the workingelectrode baseline for background signal drift due to, for example,temperature. This ability to correct the baseline signal of the workingelectrode for background drift means that the measurements of the gassensing apparatus are highly accurate. This is particularly importantwhen measuring low concentrations of NO₂ and O₃.

The additional working electrode can be made of various electrode activematerials which include carbon, gold, gold alloys, Pt alloys andplatinum. The platinum can be in the form of platinum oxide whichincludes platinum black (Ptb) and the gold can be in the form of goldoxide which includes gold black. The carbon and a carbon electrode areas described above for the first and second working electrodes. In oneembodiment, the additional working electrode is a carbon electrode.

The electrolyte is typically a liquid electrolyte, for example, dilutedH₂SO₄ (5M). Other standard electrolytes used in amperometric sensorsinclude diluted H₃PO₄ and tetraalkyl ammonium salts dissolved inpropylene carbonate. Typically, the first electrolyte, secondelectrolyte or a common electrolyte (as appropriate) are chosen fromH₂SO₄, propylene carbonate and tetraethylammonium fluoride or H₃PO₄.

The apparatus of the invention can contain control circuits that canswitch or activate/deactivate the electrodes and/or a processor forprocessing the current signals from the electrodes to thereby determinea concentration of the at least one analyte (e.g. NO₂ and/or O₃).

The optional features described above in respect of any aspect of theinvention apply to each aspect of the invention, including both theproduct and process features.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 relates to a gas sensing apparatus of the invention comprising asingle electromechanical amperometric gas sensor for detecting NO₂,having an ozone filter, working electrode and its associated counterelectrode, reference electrode, electrolyte and additional workingelectrode;

FIGS. 2A and 2B relates to a gas sensing apparatus of the inventionaccording to an embodiment comprising two individual electrochemicalamperometric gas sensors one (shown in FIG. 2A) housing an ozone filter,the first working electrode and its associated counter electrode,reference electrode, electrolyte and additional working electrode, theother (shown in FIG. 2B) housing another (the second) working electrodeand its associated counter electrode, reference electrode, electrolyteand additional working electrode;

FIG. 2C is a replacement for the gas sensor of FIG. 2B employed (alongwith the sensor of FIG. 2A) in embodiments of the invention in which asolid Mn₂O₃ layer is used as filter;

FIG. 3 is a schematic cross-sectional view of a gas sensing apparatus ofthe invention according to an embodiment wherein first and secondworking electrodes are combined into the same housing and share a commoncounter electrode, reference electrode and electrolyte;

FIG. 4 is a diagram of potentiostatic circuitry for a gas sensingapparatus in which first and second working electrodes share a commoncounter electrode, reference electrode and electrolyte as for example,in the embodiment of FIG. 1 or in which a working electrode and anadditional working electrode share a common counter electrode, referenceelectrode and electrolyte as for example, in one of the individual gassensors of the embodiment of FIG. 2;

FIG. 5 is an illustration of the experimental setup used for thetreatment of the Mn₂O₃ filter powder;

FIG. 6A shows the current response to 0.5 ppm O₃ of (a) unfilteredsensors according and (b) sensors in which the filter is 500 mg soliduntreated Mn₂O₃; FIG. 6B shows the current response to 2 ppm NO₂ of (a)a sensor in which the filter is 500 mg solid untreated Mn₂O₃ and (b) asensor in which the filter is 500 mg of solid Mn₂O₃ treated with NO₂ asdescribed above; and FIG. 6C shows the current response to 0.5 ppm O₃ of(a) unfiltered sensors and (b) sensors in which the filter is 500 mgsolid Mn₂O₃ treated with NO₂ as described above.

FIG. 7A illustrates the output current, over time, where the unfilteredsensor (trace (a)) and filtered sensor (trace (b)) are exposed in turnto zero air, then 1 ppm NO₂, then zero air, then 1 ppm O₃, then zeroair, then a mixture of 1 ppm NO₂ and 1 ppm O₃; FIG. 7B shows thecalculated O₃ concentration (trace (a)) and NO₂ concentration (trace(b)) which the traces of FIG. 7A represents;

FIG. 8 is a plot of the variation with time in cross-sensitivity to NO(as a fraction of the sensitivity to NO₂) of (a) a sensor according toFIG. 2A(a) in which the filter is formed from 500 mg of solid Mn₂O₃, and(b) in which the filter is formed from 450 mg of solid MnO₂;

FIG. 9 shows the output current with time in the presence of zero air,then 0.5 ppm O₃, then zero air of sensors according to FIG. 2A(filtered) in which the filters were formed by 25 mg of powdered Mn₂O₃mixed with 225 mg of PTFE particles having a size range of 710 μm to1500 μm (i.e. 10% by weight of Mn₂O₃ in binder).

FIG. 10A shows the current response with time to 1 ppm of O₃ of (a) asensor according to FIG. 2A (filtered) in which the ozone filtercomprised 500 mg of powdered Mn₂O₃, (not mixed with binder) and (b) asensor according to FIG. 2B (unfiltered), and in FIG. 10B (a) a sensoraccording to FIG. 2A (filtered) in which the ozone filter comprised 25mg of powdered Mn₂O₃ mixed with 225 mg of PTFE particles having a sizerange of 710 μm to 1500 μm (i.e. 10% by weight of Mn₂O₃ in binder) and(b) a sensor according to FIG. 2B (unfiltered).

FIGS. 11A-11B shows the current response to a range of concentrations ofNO₂ of sensors according to FIG. 2A (filtered) with filters comprising(A) 500 mg of powdered Mn₂O₃ and (B) 25 mg of powdered Mn₂O₃ mixed with225 mg of PTFE particles having a size range of 710 μm to 1500 μm.

FIG. 12 shows the current response to 2 ppm NO₂ of sensors according toFIG. 2A (filtered) with filters comprising, in FIG. 12A, 500 mg ofpowdered Mn₂O₃ (a) 30 days after manufacture and (b) 173 days aftermanufacture and, in FIG. 12B, 25 mg of powdered Mn₂O₃ mixed with 225 mgof PTFE particles having a size range of 710 μm to 1500 μm (i.e. 10% byweight of in Mn₂O₃ in binder), (a) 19 days after manufacture and (b) 187days after manufacture.

FIG. 13 is a photograph, to the scale shown, of filter materialaccording to the invention, in the form of PTFE particles coated withMn₂O₃ crystals.

FIG. 14(A) shows the current response with time to air and then to 0.5ppm O₃ of (a) an SO₂ sensor without an ozone filter, and (b) an SO₂sensor with an ozone filter comprising 10% by weight of Mn₂O₃ powder inbinder (after NO₂ treatment).

FIG. 14(B) shows the current response with time to air and then to 2 ppmSO₂ of (a) an SO2 sensor without an ozone filter, and (b) an SO2 sensorwith an ozone filter comprising 10% by weight of Mn2O3 powder in binder(after NO2 treatment).

FIG. 15A shows the current response with to 2 ppm NO₂ and FIG. 15B showsthe current response to 500 ppb O₃ of sensors (a) according to FIG. 2B(unfiltered) and (b) sensors according to FIG. 2A in which the filterswere formed by 40 mg of powdered Mn₂O₃ mixed with 220 mg of 500 μm PTFEparticles (i.e. 16% by weight of Mn₂O₃ in binder).

FIG. 16A shows the current response to 2 ppm NO₂ and FIG. 16B shows thecurrent response to 500 ppb O₃ of sensors (a) according to FIG. 2B(unfiltered) and (b) sensors according to FIG. 2C in which the filterswere formed by 25 mg of powdered Mn₂O₃ deposited onto a membrane (15mg·cm⁻²).

FIG. 17 shows the current response with time to a range ofconcentrations of NO₂ of sensors (a) according to FIG. 2B (unfiltered),(b) according to FIG. 2A with a filter comprising 40 mg of powderedMn₂O₃ mixed with 220 mg of 500 μm PTFE particles and (c) according toFIG. 2C with 25 mg of powdered Mn₂O₃ deposited onto a membrane (15mg·cm⁻²).

FIG. 18A is a plot of the variation with time in cross-sensitivity to O₃(as a fraction of the sensitivity to NO₂) of a sensor according to FIG.2A (filtered) containing 500 mg of powdered Mn₂O₃.

FIG. 18B is a similar plot to FIG. 18A of (a) a sensor according to FIG.2A with a filter comprising 40 mg of powdered Mn₂O₃ mixed with 220 mg of500 μm PTFE particles, (b) a sensor according to FIG. 2A with 45 mg ofpowdered Mn₂O₃ mixed with 235 mg of 100 μm PTFE particles and (c) asensor according to FIG. 2C with 100 mg of powdered Mn₂O₃ deposited ontoa membrane (30 mg·cm⁻²).

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 is a schematic cross-sectional view of gas sensing apparatusaccording to the invention in the form of a gas sensor (2) having ahousing (16), a (single) sensing working electrode (11), a counterelectrode (13), a reference electrode (14), an additional workingelectrode (35) for correcting for baseline drift, and a body orreservoir of electrolyte, mainly held in wetting filters (15). Thehousing has an inlet (17) through which the working electrode (11) isplaced in communication with a sample gas (e.g. the atmosphere), throughan ozone filter (19).

A series of circular separator discs, wicks or wetting filters (15)separate the working electrode from the counter electrode and thereference electrode. The circular separator discs, wicks or wettingfilters (15) are made of a hydrophilic, non-conductive materialpermeable to the electrolyte which functions to transport electrolyte bycapillary action. Typically the material is glass fibre. The circularseparator discs, wicks or wetting filters serve to ensure that each ofthe electrodes is in contact with the electrolyte.

The working electrode (11) is a carbon electrode and typically comprisea catalytic layer of carbon mixed with polytetrafluoroethylene (PTFE)binder which is then bonded to a gas porous, but hydrophobic, PTFEsupport to allow gas support to the catalyst, i.e. the electrode activematerial, but avoid electrolyte leakage or flooding of the electrode.The carbon electrodes can be manufactured using common conventionaltechnologies such as pressing, screen printing, inkjetting and sprayinga carbon slurry onto a porous membrane. Here the working electrodecatalyst will typically have a diameter of 14 mm. A mixture of carbonand microparticulate polytetrafluoroethylene (PTFE) is sintered and ispreferably prepared by pressing the resulting mixture onto a support inthe form of a gas porous membrane, such as PTFE sheet. Where carbon ispressed at the normal pressure used in the field, i.e. around 200kg/cm², the amount of catalyst is preferably between 5 and 30 mg per cm²of electrode surface area. Preferably the binder is a Fluon matrix(Fluon is a Trade Mark) of around 0.002 ml per cm². Other electrodesused in the gas sensing apparatus of the invention, such as the Platinumblack electrodes can be prepared in a similar way.

An O ring is located at the top of the porous membrane (21) and acts toseal the sensor and to aid compressing the stack of components when thesensor is sealed. Also present are a number of platinum strips thatserve to connect each electrode to one of the terminal pins (24)provided at the base of the sensor. Closing housing is a dust filter(25) to prevent dust and other foreign objects from entering.

The ozone filter (19) is formed of MnO₂ powder which has been treatedwith NO₂ as described below and the sensor is useful to sense NO₂ in thepresence of gases such as SO₂, CO, NO, NH₃, H₂ and O₃, which were it notfor the choice of electrode and the presence of the ozone filter, wouldotherwise interfere with the measurement.

FIGS. 2A and 2B are together a schematic cross-sectional diagram of gassensing apparatus according to the invention in the form of twoindividual gas sensors (2) (shown in FIG. 2A) and (4) (shown in FIG.2B), each having its own housing (16), working electrode, counterelectrode (13), reference electrode (14), additional electrode (35) andelectrolyte held mainly in wetting filters (15). Together, these form agas sensing apparatus (30) according to the invention. These gas sensorshave similar stacked structures.

The first gas sensor (2) has an ozone filter (19) in internal chamber(23) above the working electrode (11A), functioning as the first workingelectrode. Again, the ozone filter is prepared by the method set outbelow. The second gas sensor (4) has a working electrode (11B), which isthe same as the working electrode (11A) of the first gas sensor (4) butfunctions as the second working electrode, and has no ozone filter inthe internal chamber (23). The two working electrodes (11A), (11B)correspond to those described above in relation to FIG. 1.

The embodiment of FIGS. 2A and 2B is especially useful for detecting NO₂in the presence of ozone, or for detecting ozone, or both. This isbecause ozone is removed from the sample gas which penetrates the inletof the first gas sensor (20), before NO₂ is sensed by the workingelectrode, and so the first gas sensor gives a measurement of theconcentration of NO₂ only, but ozone is not filtered from the gasreceived through the inlet of the second gas sensor (22) and so thesecond gas sensor gives a signal which is indicate of the concentrationof both NO₂ and ozone. The two signals can therefore be processed toindependently determine the concentration of NO₂ and ozone and thereforeto measure either NO₂, or ozone, or both.

In some of the embodiments discussed below, the sensor of FIG. 2A isreplaced with the sensor of FIG. 2C. The sensor of FIG. 2C correspondsto the sensor of FIG. 2A except that there is no filter powder receivedin the chamber (23) and instead there is an additional microporous PTFEmembrane (25) having a microporous solid filter layer (27) comprisingMn₂O₃ formed thereon between the inlet (17) and the first workingelectrode (11A). The filter layer can be formed on the inward or outwardsurface of the membrane (25). In some embodiments, there is a Mn2O3powder present in the chamber (23) in addition to the solid filter layer(27).

FIG. 3 is a schematic cross-sectional view of a further embodiment ofgas sensing apparatus (6) in which the first (11A) and second (12A)working electrodes share a common counter electrode (13), a commonreference electrode (14) and a common body or reservoir of electrolyte,again mainly held in wetting filters (15). In this case, the housing(16) has two inlets (17) and (18), which place the second workingelectrode (11) and the O₃ filter (19) in direct communication with thesample gas (e.g. the atmosphere) in parallel. Inlets (17) and (18) canbe capillary inlets, i.e. inlets which are sized so that they controlthe rate of sample gas reaching the electrodes so that the gas sensingapparatus is diffusion limited, A central portion (20) divides a cavitydefined by the housing (16) and a porous membrane (21) into first (22)and second (23) internal chambers. The first and second workingelectrodes are located in the same horizontal plane and are situatedunderneath the porous membrane (21). The second working electrode (11B)is situated underneath the first internal chamber (22) and the firstworking electrode (12) is situated underneath the second internalchamber (23). The ozone filter (19) is located in the second internalchamber (23), adjacent/on top of part of the porous membrane (21)covering the first working electrode (12). The ozone filter (19) coversthe surface of the first working electrode (12) that would, absent thefilter, be exposed to the sample gas.

Example circuitry for the gas sensing apparatus of the invention in theembodiment of FIG. 3 is shown in FIG. 4, where WE1 is the second workingelectrode (11B) and WE2 is the first working electrode (11A). Thiscircuitry could also be used for either of the individual sensors makingup the gas sensing apparatus of the invention in the embodiment of FIG.2, where, for example, WE1 is the second working electrode and WE2 isthe first additional working electrode, or WE1 is the first workingelectrode and WE2 is the second additional working electrode. It couldalso be used for a simple individual sensor according to FIG. 2A (or 2B)but lacking an additional working electrode in which case WE1 is theworking electrode and is the only one recorded.

Forming the Ozone Filter

In order to form the ozone filter, Mn₂O₃ filter powder (99% purity) istreated by passing a flow of NO₂ gas therethrough. With reference toFIG. 5, the Mn₂O₃ filter powder is inserted into a gas washing bottle(or Drechsel's bottle) which is then connected through the inlet to asource of NO₂ gas. The Mn₂O₃ may first be also be mixed with PTFEpowder, acting as binder.

In an illustrative example, 20 g of Mn₂O₃ or 30 g or 50 g of Mn₂O₃/PTFEfilter powder was treated in a 250 ml gas washing bottle. The NO₂ gaswas passed through the filter powder at a concentration of 100 ppm and aflow rate of 0.5 l·min⁻¹ for 2 hours.

By monitoring the concentration of NO₂ gas leaving the bottle, wetherefore calculated the amount of NO₂ which was adsorbed before theMn₂O₃ was saturated. This experiment was repeated. It was found that1.78×10⁻⁸ moles of NO₂ were adsorbed per gram of Mn₂O₃. Using the valueof surface area of the MnO₃ powder obtained from BET analysis of 2.42m²/g we estimated that 2.41×10¹² molecules of NO₂ were adsorbed per cm²of Mn₂O₃ surface. This is consistent with the values repeated in thepaper J. Phys. Chem. C 2014, 118, 23011-23021 where the number of NO₂molecules adsorbed on TiO₂ are in the order of 10¹³ molecules/cm².

The resulting material is advantageous in that it filters ozone withminimal degradation of the NO₂ signal compared to untreated Mn₂O₃. Thelayer remains gas permeable. Furthermore, it filters ozone efficiently,enabling a relatively low proportion of Mn₂O₃ mixed with PTFE tofunction effectively, reducing cost. An image of the resulting Mn₂O₃coated PTFE particles is shown in FIG. 13.

A mixture of Mn₂O₃ and PTFE particles having a size of having a sizerange of 710 μm to 1500 μm was prepared by manually mixing Mn₂O₃ powderwith PTFE in a glass container until all Mn₂O₃ powder coats the PTFE. Insome of the examples below, Mn₂O₃ and PTFE were mixed in a weight ratioof 1:10. The resultant mixture is then sieved through a 710 micron sievestack using a motorised mechanical sieve for 1 hour. The portion thatdid not fall through the 710 micron sieve is then collected while theremaining material that did fall through the 710 micron sieve isdiscarded.

In some examples below, Mn₂O₃ and PTFE were also mixed in a weight ratioof 1.6:8.4 (16% by weight). In those examples, the resultant mixture wasthen sieved through a 500 micron sieve stack using a motorisedmechanical sieve for 1 hour. The portion that did not fall through the500 micron sieve was then collected while the remaining material thatdid fall through the 500 micron sieve was discarded.

In the examples where the Mn₂O₃ was mixed with 100 micron PTFE in aweight ratio of 1.6:8.4 (16% by weight), resultant mixture was thenthoroughly shaken using a motorised mechanical sieve for 1 hour.

Filter in the Form of a Solid Layer of Powdered Mn₂O₃:

For examples 13 to 15, solid filter layers of powdered Mn₂O₃ were formedwith a diameter of 14 mm and 19 mm. A mixture of powdered Mn₂O₃ andmicroparticulate polytetrafluoroethylene (PTFE) was sintered and theresulting mixture was pressed onto a support PTFE sheet, acting as a gasporous membrane. Mn₂O₃ was pressed at the normal pressure used in thefield, i.e. around 400-600 kg/cm², and the amount of Mn₂O₃ was in therange 15 to 30 mg per cm² of surface area. The Mn₂O₃ was mixed with aFluon matrix (Fluon is a Trade Mark) of around 0.0065 ml per cm², whichcontains PTFE which acted as binder. In these examples Fluon with a PTFEparticle size in the range of 200-300 microns diameter was used. Mn₂O₃was 74% by weight of the solid microporous layer, with the balance beingPTFE.

EXPERIMENTAL SECTION

The sensors used in the following experiments were tested on standardpotentiostatic circuit boards (FIG. 4). Generally, the sensors werestabilised for a minimum of 2 days before being tested. All theexperiments involving gas tests were controlled using computercontrolled valves and digital mass flow controllers. Sensor output datacollection is also made using a computer. The NO₂ gas tests were madeusing a certified 100 ppm bottle (Air Products, UK) and filtered air,except that a 10 ppm bottle (Air Products, UK) was used for Experiment 2and the linearity test of Experiment 7. The ozone was obtained using acalibrated ozone generator equipped with an internal analyser (ThermoScientific, Model 49i-PS), except for Experiment 2 and the filtercapacity test of Experiment 5 where the ozone was obtained using acalibrated ozone generator (Ultra-violet Products Ltd, SOG-1, Cambridge,UK). During the laboratory tests the sensors were exposed to a gas flowof 0.5 l·min⁻¹.—

The following materials were used in the filter powder. These materialswere used in each following experiment unless indicated to the contrary.

Manganese (III) oxide (Mn₂O₃), 99% purity, approx. 325 mesh (44micrometers) powder, from Sigma Aldrich, product number 377457. BETanalysis on the sample gives a surface area of 2.242 m²/g.

Manganese (IV) oxide (MnO₂), 99.9% purity, approx. 325 mesh (44micrometers) powder, from Alpha Aesar, product number 42250. BETanalysis on the sample gives a surface area of 2.08 m²/g.

PTFE binder (Fluon PTFE G307 (median particle size 500 to 1500 microns)(Fluon is a trade mark). The powder was sieved to collect only particleswith a size of at least 710 microns. Also: Fluon PTFE G201 (medianparticle size 500 microns) and Fluon PTFE G204 (median particle size 100microns).

The working electrodes were made from carbon graphite (particle size <20μm, Aldrich, product code 282863).

Calibration of the sensors according to the invention was carried out asfollows. The sensor output is given in nA. Amperometric gas sensors havea linear output with target gas analyte concentration. This makespossible to use a simple calibration procedure where the relationbetween the sensor output and the gas concentration is determined byexposing the sensor to a known concentration of gas analyte. For thisapplication the sensitivity to both the first and the first workingelectrodes is determined for each of the target gases, NO₂ and O₃. Thesensor output can then be used to calculate the concentration of NO₂ andO₃.

If the following parameters are defined as follows:

-   i₁ is the current observed on the second working electrode.-   I₂ is the current observed on the first working electrode.-   S_(1(NO2)) is the second working electrode sensitivity to NO₂.-   S_(1(O3)) is the second working electrode sensitivity to O₃.-   S_(2(NO2)) is the first working electrode sensitivity to NO₂.-   S_(2(O3)) is the first working electrode sensitivity to O₃.-   C_((NO2)) is the NO₂ analyte concentration to determine.-   C_((O3)) is the O₃ analyte concentration to determine.

Then, by definition and taking into account the linear relationshipbetween the sensor output and the analyte concentration, the followingcan be written:

i ₁ =S _(1(NO2)) ·C _((NO2)) +S _(1(O3)) ·C _((O3))

i ₂ =S _(2(NO2)) ·C _((NO2)) +S _(2(O3)) ·C _((O3))

The ozone filter on top of the first working electrode means thatS_(2(O3))=0. So C_((NO2)) can be calculated using the simple relation:

C _((NO2)) =i ₂ /S _(2(NO2))

The NO₂ analyte concentration being now known it is then possible tocalculate the O₃ analyte concentration using the second workingelectrode output:

C _((O3))=(i ₁ −S _(1(NO2)) ·C _((NO2)))/S _(1(O3))

or C _((O3))=(i ₁ −S _(1(NO2))·(i ₂ /S _(2(NO2))))/S _(1(O3))

Experiment 1

In a first example, sensors according to FIG. 2A (filtered) were formedwith Mn₂O₃, 99% purity (without PTFE) with and without the nitrogendioxide pretreatment step described above. The response of these sensorsand sensors according to FIG. 2B (i.e. unfiltered) to ozone and NO₂ wascompared. The unfiltered sensor is commercially available under thetrade name OX-A421, manufactured and sold by Alphasense Limited of GreatNotley, United Kingdom.

For these and subsequent experiments the first working electrode, thesecond working electrode, the additional electrode and the referenceelectrode are made of carbon graphite and the counter electrode is madeof platinum black.

FIG. 6A shows the current response to 0.5 ppm O₃ of (a) unfilteredsensors and (b) sensors in which the filter is 500 mg of untreatedMn₂O₃.

FIG. 6B shows the current response to 2 ppm NO₂ of (a) a sensor in whichthe filter is 500 mg of untreated Mn₂O₃ and (b) a sensor in which thefilter is 500 mg of Mn₂O₃ treated with NO₂ as described above.

FIG. 6C shows the current response to 0.5 ppm O₃ of (a) unfilteredsensors and (b) sensors in which the filter is 500 mg of Mn₂O₃ treatedwith NO₂ as described above.

It is apparent from the results that the untreated Mn₂O₃ filterefficiently removes O₃. No signal is observed in the presence of 500 ppbof O₃ (FIG. 6A, curve (b)) but a sensor which differs only by theomission of the filter gives a clearly defined current response.However, it can be seen that the untreated filters added to the sensorsto remove O₃ do affect the NO₂ signal. It is apparent from FIG. 6B,curve (a) that the resulting signal is not adequate for reliable sensingof NO₂. However, we have found that a well-defined signal for NO₂ can beobtained with the NO₂ pretreatment step set out above. It can be seenfrom FIG. 6C, curve (b), that the treated Mn₂O₃ remains effective forthe filtering of NO₂.

Experiment 2

In this example, sensing apparatus comprised a sensor according to FIG.2A in which the filter (59) in the chamber (52) was formed with 500 mgMn₂O₃, (without PTFE), and a sensor according to FIG. 2B (unfiltered).

For this and all subsequent experiments, the Mn₂O₃ had been treated withNO₂ gas described above.

FIG. 7A illustrates the output current, over time, where the unfilteredsensor (trace (a)) and filtered sensor (trace (b)) are exposed in turnto zero air (zero air is air that has been filtered to remove most gases(NO₂, NO, CO, SO₂, O₃, etc.) and is used as a calibrating gas for zeroconcentration), then 1 ppm NO₂, then zero air, then 1 ppm O₃ and thenzero air and then a mixture of 1 ppm NO₂ and 1 ppm O₃. FIG. 7B shows thecalculated O₃ concentration (trace (a)) and NO₂ concentration (trace(b)) which these traces represent.

This figure shows that with a mixture of NO₂ and O₃, the filtered sensorsenses only NO₂, whereas the unfiltered sensor detects both gases and,in the presence of both gases simultaneously, the output of theunfiltered sensor corresponds to the sum of the output expected for eachof the gases. Accordingly, Mn₂O₃ is suitable as a filter material toremove ozone without affecting the signal due to NO₂.

Experiment 3

Sensors according to FIG. 2A (filtered) were formed with filterscomprising (a) 500 mg of Mn₂O₃, and (b) 450 mg of MnO₂ were exposed to 2ppm NO₂ for 10 minutes and then to 2 ppm nitrogen monoxide NO for 10minutes. FIG. 8 is a plot of the variation with time incross-sensitivity of the sensors (a) and (b) to NO, i.e. the currentresponse to NO as a fraction of the current response to a correspondingconcentration (in this case ppm) of NO₂.

This shows that the cross-sensitivity to NO is systematically lower withMn₂O₃ than MnO₂, and that this improvement persists.

Experiment 4

Sensors according to FIG. 2A (filtered) were formed with ozone filterscomprising mg of powdered Mn₂O₃ mixed with 225 mg of PTFE particleshaving a size range of 710 μm to 1500 μm (i.e. 10% by weight of in Mn₂O₃binder), and exposed to zero air, then 0.5 ppm O₃, then zero air. FIG. 9shows the output current with time and demonstrates that ozone isefficiently filtered out by Mn₂O₃ mixed with PTFE binder (10% by weightof Mn₂O₃ in PTFE binder particles).

Experiment 5

The ozone filtering capacity of the sensors was further tested and FIG.10 shows the current response to 1 ppm of O₃ (2 ppm of O₃ is relativelyhigh in comparison to levels typically measured in environmentalmonitoring) of in FIG. 10A (a) a sensor according to FIG. 2A (filtered)in which the ozone filter comprised 500 mg of powdered Mn₂O₃ (not mixedwith binder) and (b) a sensor according to FIG. 2B (unfiltered), and inFIG. 10B (a) a sensor according to FIG. 2A (filtered) in which the ozonefilter comprised 25 mg of powdered Mn₂O₃ mixed with 225 mg of PTFEparticles having a size range of 710 μm to 1500 μm (i.e. 10% by weightof in Mn₂O₃ binder) and (b) a sensor according to FIG. 2B (unfiltered).In the case of FIG. 10A the experiment was continued for 14 days and inthe case of FIG. 10B for 10 hours.

These results again demonstrate that Mn₂O₃ powder, whether unmixed ormixed with mg of PTFE particles having a size range of 710 μm to 1500 μmforms an efficient ozone filter.

Experiment 6

Experiments were carried out to assess the cross sensitivity of thesensors to common interferents. Table 1, below, shows the crosssensitivity of sensors formed with (a) mg of powdered Mn₂O₃ and (b) 25mg of powdered Mn₂O₃ mixed with 225 mg of PTFE particles having a sizerange of 710 μm to 1500 μm (i.e. 10% by weight of Mn₂O₃ in binder) tospecific gases, relative to NO₂, at the concentrations stated. Twosensors of each type were formed and tested.

TABLE 1 500 mg Mn₂O₃ 25 mg Mn₂O₃ GAS ppm Sensor 1 Sensor 2 Sensor 1Sensor 2 SO2 5 1.53% 0.81% 0.86% 0.79% CO 5 1.25% 0.85% 1.41% 1.39% H2100 0.24% 0.04% 0.06% 0.00% CO2 50000 0.00% 0.00% 0.00% 0.00% NH3 200.42% 0.18% 0.09% 0.06%

Further experiments demonstrated that the quality of ozone filtering wasunsatisfactory if the Mn₂O₃ powder was reduced to 5% or less by mass ofthe combined mixture of Mn₂O₃ and PTFE binder particles. Mixture ofMn₂O₃ powder with at least the same mass, and ideally more, of PTFEparticles leads to the Mn₂O₃ coating the particles rather than formingsolid masses.

For unmixed Mn₂O₃ powder (as specified in the Experimental Sectionabove) we found that the ratio of the active surface area of filtermaterial (by BET analysis) to the cross-sectional area of the filter(perpendicular to the gas path through the filter) was about 0.85 m² percm². For Mn₂O₃ mixed with PTFE binder to 10% by mass of the combinedmixture of Mn₂O₃ and binder, this ratio was 0.025 m² per cm² and forMn₂O₃ mixed with PTFE binder to 8% by mass of the combined mixture itwas 0.02 m² per cm².

Experiment 7

FIG. 11 shows the current response to a range of concentrations of NO₂of sensors according to FIG. 2A (filtered) with filters comprising (A)500 mg of powdered Mn₂O₃ and (B) 25 mg of powdered Mn₂O₃ mixed with 225mg of PTFE particles having a size range of 710 μm to 1500 μm (i.e. 10%by weight of Mn₂O₃ in binder). The results show good sensitivity andlinearity with NO₂ concentration with both the mixed with binder andunmixed filters.

Experiment 8

FIG. 12 shows the current response to 2 ppm NO₂ of sensors according toFIG. 2A (filtered) with filters comprising, in FIG. 12A, 500 mg ofpowdered Mn₂O₃ (a) 30 days after manufacture and (b) 173 days aftermanufacture and, in FIG. 12B, 25 mg of powdered Mn₂O₃ mixed with 225 mgof PTFE particles having a size range of 710 μm to 1500 μm (i.e. 10% byweight of Mn₂O₃ in binder), (a) 19 days after manufacture and (b) 187days after manufacture.

The results demonstrate that sensors using NO₂ treated Mn₂O₃ powder,with or without dilution, shows a good and reproducible response to NO₂6 months after NO₂ treatment.

Experiment 9

Table 2 below shows the change with time (in days) of the crosssensitivity to 2 ppm NO (i.e. the ratio of the current response to 2 ppmNO to the current response to 2 ppm NO₂) of sensors according to FIG. 2A(filtered) with filters comprising 25 mg of powdered MnO₂ mixed with 225mg of the PTFE particles having a size range of 710 μm to 1500 μm; 450mg of powdered MnO₂ (not mixed with binder), 25 mg of powdered Mn₂O₃mixed with 225 mg of PTFE particles having a size range of 710 μm to1500 μm (i.e. 10% by weight of Mn₂O₃ in binder), and 500 mg of Mn₂O₃(i.e. not mixed with binder) (a) 19 days after manufacture and (b) 187days after manufacture.

TABLE 2 Age 25 mg 450 mg 25 mg 500 mg (days) ppm MnO₂ MnO₂ Mn₂O₃ Mn₂O₃10 2 1.6 9.7 1.3 1.7 110 2 5.7 13.9 2.2 4.5 210 2 9.2 13.3 4.4 5.7

This shows that the cross-sensitivity to NO is lower when the ozonefilter comprises powdered Mn₂O₃ than when the ozone filter comprisespowdered MnO₂, and that the sensors in which Mn₂O₃ is mixed with PTFEgive the best (lowest) cross-sensitivity to NO. Of particular benefit,the NO cross-sensitivity increases with time but this deterioration isreduced with powdered Mn₂O₃ in comparison to powdered MnO₂ and isminimised when powdered Mn₂O₃ is mixed with PTFE.

Experiment 10

Experiments were carried out to assess the temperature sensitivity ofsensors according to FIG. 2B (unfiltered) (columns 2 and 3 below), andaccording to FIG. 2A (filtered) in which the ozone filter was 10% byweight of Mn₂O₃ powder in binder (after NO₂ treatment), 10% by weight ofMn₂O₃ powder in binder (without NO₂ treatment) and NO₂ treated Mn₂O₃powder without binder (columns 4 through 6 below, respectively). Thecurrent response due to 2 ppm NO₂ at different temperatures, as apercentage of the current response due to 2 ppm NO₂ at 20° C. is shownin Table 3 below. The results show that, surprisingly, the ozone filterin which Mn₂O₃ powder was pretreated with NO₂ and mixed with PTFE has amuch better current response at low temperatures than unmixed Mn₂O₃powder and accordingly has a substantially better operating temperaturerange.

TABLE 3 10 w/w % 10 w/w % 100 w/w % Temp. No No Mn₂O₃ NO₂ Mn₂O₃ notMn₂O₃ NO₂ ° C. filter filter treated treated treated −30 70 68 70 39 −2−20 75 73 80 56 6 −10 82 79 88 75 45 0 89 87 94 90 85 10 95 94 98 98 9720 100 100 100 100 100 30 105 104 104 101 103 40 110 107 103 99 107 50112 117 102 100 128

Experiment 11

In order to demonstrate that the ozone filter is useful with analytesother than NO₂, we modified a commercial electrochemical sensor for SO₂,having gold/ruthenium working and reference electrodes and a platinumblack counter electrode (brand SO2-A4 available from Alphasense Limited,Great Notley, UK) by replacing an H₂S filter with 25 mg of Mn₂O₃ powdermixed with 225 mg PTFE particles having a size range of 710 μm to 1500μm, treated with NO₂ as above.

FIG. 14(A) shows the current response with time to air and then to 0.5ppm O₃ of (a) an SO₂ sensor without an ozone filter, and (b) an SO₂sensor with an ozone filter comprising 10% by weight of Mn₂O₃ powder inbinder (after NO₂ treatment).

FIG. 14(B) shows the current response with time to air and then to 2 ppmSO₂ of (a) an SO₂ sensor without an ozone filter, and (b) an SO₂ sensorwith an ozone filter comprising 10% by weight of Mn₂O₃ powder in binder(after NO₂ treatment).

The results demonstrate that the ozone filter can be used to removeozone from a gas sample in an SO₂ sensor. Again, O₃ may be measured witha first sensor which has an electrode which is sensitive to SO₂ and O₃and ozone filter according to the invention and a second sensor, alsohaving an electrode which is sensitive to SO₂ and O₃, but no ozonefilter, and comprising output signals.

Experiment 12

FIG. 15A shows the current response with to 2 ppm NO₂ and FIG. 15B showsthe current response with time to 500 ppb O₃ of sensors (a) according toFIG. 2B (unfiltered) and (b) sensors according to FIG. 2A in which thefilters were formed by 41.6 mg of powdered Mn₂O₃ mixed with 218.4 mg of500 μm PTFE particles (i.e. 16% by weight of Mn₂O₃ in binder) andpretreated with NO₂ as described above.

No signal is observed in the presence of 500 ppb of O₃ (FIG. 15B, curve(b)) but a sensor which differs only by the omission of the filter givesa clearly defined current response (FIG. 15B, curve (a)). Thus we havefound that a well-defined signal for NO₂ can be obtained with thesensors in which the filters were formed by 40 mg of powdered Mn₂O₃mixed with 220 mg of 500 μm PTFE particles.

Experiment 13

FIG. 16A shows the current response with time to 2 ppm NO₂ and FIG. 16Bshows the current response with time to 500 ppb O₃ of sensors (a)according to FIG. 2B (unfiltered) and (b) sensors according to FIG. 2Cin which the filters were formed by 25 mg of powdered Mn₂O₃ mixed withFluon (Fluon is a trade mark) and deposited onto a membrane (15mg·cm⁻²).

Fluon comprises PTFE and a solvent and the resulting solid layer hasabout 26% PTFE by mass. Most of the Mn₂O₃ is therefore not associatedwith the PTFE particles and the purpose of the PTFE is to provide someporosity, as well as to make the material easier to handle in amanufacturing setting than unmixed Mn₂O₃.

No signal is observed in the presence of 500 ppb of O₃ (FIG. 16B, curve(b)) but a sensor which differs only by the omission of the filter givesa clearly defined current response (FIG. 16B, curve (a)). We havetherefore found that a well-defined signal for NO₂ can be obtained withthe sensors in which the filters were formed by a solid layer of 25 mgof powdered Mn₂O₃ deposited onto a membrane (15 mg·cm⁻²).

Experiment 14

FIG. 17 shows the current response to a range of concentrations of NO₂of sensors (a) according to FIG. 2B (unfiltered), (b) according to FIG.2A with a filter comprising 40 mg of powdered Mn₂O₃ mixed with 220 mg of500 μm PTFE particles and (c) according to FIG. 2C with 25 mg ofpowdered Mn₂O₃ (again with about 26% PTFE by mass) and deposited onto amembrane (15 mg·cm⁻²).

This demonstrates that powdered Mn₂O₃ pressed into a solid layer mayform an effective ozone filter.

Experiment 15

Sensors according to FIG. 2A (filtered) were formed with filterscomprising 500 mg of Mn₂O₃(FIG. 18A) and (a) 40 mg of powdered Mn₂O₃mixed with 220 mg of 500 μm PTFE particles, (b) 45 mg of powdered Mn₂O₃mixed with 235 mg of 100 μm PTFE particles and (c) 100 mg of powderedMn₂O₃, mixed with Fluon (Fluon is a trade mark), and deposited onto amembrane (30 mg·cm⁻²) (FIG. 18B). In this case, the mass fraction of thedeposited layer formed by PTFE is 15%. In contrast to Experiment 13, theMn₂O₃ was pretreated with NO₂ by the process described above.

The sensors were exposed to 2 ppm NO₂ for 10 minutes and then to 500 ppbozone O₃ for 5 minutes. FIG. 18 is a plot of the variation with time incross-sensitivity of the sensors to O₃, i.e. the current response to O₃as a fraction of the current response to a corresponding concentration(in this case 2 ppm) of NO₂.

These results demonstrate that Mn₂O₃ powder, whether unmixed, mixed withvarious PTFE particles sizes or as a solid layer deposited onto amembrane, forms an efficient ozone filter.

Although in the examples shown which use a solid layer, the layercomprises some PTFE in addition to Mn₂O₃, it is provided simply toensure that the solid layer is microporous and can therefore bepenetrated by analyte gas. Alternatively, Mn₂O₃ may be deposited byother means to give a microporous structure, for example by depositingMn₂O₃. microparticles without binder, or screen printing optionally withglass or other particles. The PTFE is not required.

Furthermore, in further example embodiments, Mn₂O₃ powder, whetherunmixed, or mixed with binder as described above, may be used incombination with a microporous solid Mn₂O₃ layer, to give a more robustsensor, for example for use in atmospheres with a particularly highconcentration of O₃.

CONCLUSIONS

In the case of gas sensing apparatus for detecting NO₂ and/or O₃, theresults demonstrate that Mn₂O₃ is useful as an ozone filter, whether asa powder or as a solid microporous layer. The cross sensitivity to NOcan be reduced by mixing the Mn₂O₃ powder with binder, or by treating itwith sufficient NO₂. Although these mixing or treatment steps couldcompromise ozone filtering, we have found that they have a greatereffect in reducing cross sensitivity to NO and that it is thereforepossible to obtain an efficient ozone filter with low cross sensitivityto NO.

What is claimed is:
 1. Electrochemical gas sensing apparatus for sensingat least one gaseous analyte and comprising a housing having an inlet, asensing electrode and an ozone filter interposed between the sensingelectrode and the inlet, wherein the ozone filter comprises Mn₂O₃. 2.Electrochemical gas sensing apparatus according to claim 1, wherein theMn₂O₃ is Mn₂O₃ powder.
 3. Electrochemical gas sensing apparatusaccording to claim 1 wherein the Mn₂O₃ is NO₂ treated. 4.Electrochemical gas sensing apparatus according to claim 3, wherein theMn₂O₃ is Mn₂O₃ powder which has at least 10¹² molecules of NO₂ adsorbedthereto per cm² of surface.
 5. Electrochemical gas sensing apparatusaccording to claim 2, wherein the ozone filter comprises Mn₂O₃ powdermixed with binder.
 6. Electrochemical gas sensing apparatus according toclaim 5, wherein the ozone filter comprises binder particles coated withthe Mn₂O₃ powder.
 7. Electrochemical gas sensing apparatus according toclaim 1, wherein the ozone filter comprises a solid microporous layerwhich comprises Mn₂O₃.
 8. Electrochemical gas sensing apparatusaccording to claim 1 for sensing NO₂ and/or ozone.
 9. Electrochemicalgas sensing apparatus according to claim 1, further comprising a secondsensing electrode, wherein the second working electrode is in gaseouscommunication with a gas sample without an intervening ozone filter. 10.Electrochemical gas sensing apparatus according to claim 9, wherein theat least one said analyte comprises ozone.
 11. A method of forming anelectrochemical gas sensing apparatus for sensing at least one gaseousanalyte, the method comprising providing a housing having an inlet and asensing electrode and providing an ozone filter interposed between thesensing electrode and the inlet, wherein the ozone filter comprisesMn₂O₃.
 12. A method of forming an electrochemical gas sensing apparatusaccording to claim 11, wherein the ozone filter comprises Mn₂O₃ powder.13. A method of forming an electrochemical gas sensing apparatusaccording to claim 12, comprising the step of treating the Mn₂O₃ powderwith NO₂.
 14. A method of forming an electrochemical gas sensingapparatus according to claim 11, wherein the Mn₂O₃ is Mn₂O₃ powder ismixed with binder particles.
 15. A method according to claim 14 whereinthe Mn₂O₃ powder coats the binder particles.
 16. A method of forming anelectrochemical gas sensing apparatus according to claim 11, wherein thefilter comprises a solid microporous layer of Mn₂O₃.
 17. A method offorming an electrochemical gas sensing apparatus according to claim 16,wherein the method comprises depositing a layer of Mn₂O₃ on a gas porousmembrane.
 18. A method of forming an electrochemical gas sensingapparatus according to claim 11, comprising providing a second sensingelectrode in gaseous communication with a gas sample without anintervening ozone filter.
 19. A method of sensing NO₂ and/or ozonecomprising forming an electrochemical gas sensing apparatus by themethod of claim 18, and exposing the gas sensing apparatus to a gassample, wherein the difference in the signals from the sensingelectrodes is representative of ozone in the gas sample, and therebydetermining from the signals from the first and second sensingelectrodes the concentration of NO₂ and/or ozone in the gas sample. 20.A method of sensing NO₂ and/or ozone comprising forming anelectrochemical gas sensing apparatus by the method of claim 18, andexposing the gas sensing apparatus to a gas sample, wherein thedifference in the signals from the sensing electrodes is representativeof ozone in the gas sample, and thereby determining from the signalsfrom the first and second sensing electrodes the concentration of NO₂and/or ozone in the gas sample.
 21. A method of sensing NO₂ and/or ozonecomprising forming electrochemical gas sensing apparatus according toclaim 1 and bringing the inlet into gaseous communication with a gassample.
 22. A method of sensing NO₂ and/or ozone comprising formingelectrochemical gas sensing apparatus by the method of claim 11 andbringing the inlet into gaseous communication with a gas sample.